Problem: Simplify the following expression: ${2a-3(-a-7)}$
Explanation: Distribute the ${-3}$ into the parentheses: $ 2a {-3(}\gray{-a-7}{)} $ $ 2a + {3a+21} $ Combine the $a$ terms: $ {2a + 3a} + 21$ $ {5a} + 21$ The simplified expression is $5a+21$